Bipolar transistors are known in the art. Metal Insulator Semiconductor (MIS) bipolar transistors are known in the art. A prior art MIS transistor is represented in FIG. 1. A very thin layer of insulator 50 such as SiO.sub.2, between a metal emitter 52 and semiconductor base 54 prevents base to emitter current flow. However, when V.sub.be is large enough, hot electrons from the emitter tunnel through the insulator 50 to the base 54. This voltage is called the tunnel voltage and the current is called tunnel current. Since the hot electrons forced through the insulator are high energy electrons, they are "ballistic" as opposed to drift electrons. These ballistic electrons traverse the base 54 so quickly that, they seldom recombine with holes before entering the collector 56. Thus, MIS transistors have increased DC current gain over prior art bipolar transistors because of attenuated recombination in the base 54. However, because the emitter is still a source of drift electrons, prior art MIS transistors still suffer from carrier recombination in base 54.
Consequently, minimizing this source of drift electrons is a primary consideration in fabricating MIS transistors. In this prior art MIS transistor, the collector 56 is an n-type epitaxial layer on an n-type substrate 58. Metal base contacts 60 are isolated from the emitter 52 by thick field oxide 62. A backside contact 64 provides voltage to the collector 56 through substrate 58. To maximize DC current gain, the base dopant level is high, maximizing the base and the collector shunt resistance. If, however, the base dopant is too high, base to collector capacitance is excessive and attenuates AC current gain.
Heterojunction bipolar transistors (HBTs) are known in the art. A heterojunction has one or more layers of at least two dissimilar semiconductor materials that have different energy at the conduction and valence band edges (E.sub.c and E.sub.v respectively) and different electron affinities. Thus, the heterojunction layer interface creates band gap spikes .DELTA.E.sub.c and .DELTA.E.sub.v. .DELTA.E.sub.c and .DELTA.E.sub.v reinforce current flow in one (forward biased) direction and reduce current flow in the other (reverse biased) direction. Because the electrons effectively tunnel through the conduction potential spike, .DELTA.E.sub.c, this increased forward current flow is also known as tunnel current. Consequently, unlike (homojunction) bipolar transistors, because .DELTA.E.sub.c and .DELTA.E.sub.v reinforce DC current flow in one direction and reduce it in the other, an HBT's emitter and collector are not considered interchangeable. Thus, properly biased HBTs have an increased DC current gain over traditional homojunction bipolar transistors.
However, because HBTs suffer from effects normally ignored for homojunction bipolar transistors, instead of 2-4 orders of magnitude current improvement, prior art HBTs only have, at best, slightly more than one order of magnitude. DC current gain is impaired, partially, because the conduction band discontinuity .DELTA.E.sub.c is not as large as the valence band discontinuity .DELTA.E.sub.v. Since hole mobility is significantly lower than electron mobility, .DELTA.E.sub.v reduces hole injection from the base to the emitter and collector significantly; whereas, .DELTA.E.sub.c is insufficient to affect more than a slight reduction in electron injection from the emitter into the base. Consequently, DC current gain improvement is limited.
Additionally, DC current gain improvement is impeded by current leakage caused by heterojunction layer lattice mismatch, e.g., between Si and SiGe. During device processing, these heterojunction layers may intermix or, dislocations may form at the interface. Intermixing tends to homogenize the heterojunction, smoothing the band discontinuities and reducing .DELTA.E.sub.c and .DELTA.E.sub.v. If intermixing occurs, neither hole nor electron current is minimized. A dislocation in the HBT's active area acts as a diffusion pipe, i.e., a leakage path. Dislocations also form electron traps in the HBT's base-emitter interface, base, and base-collector interface regions. These traps lead to unwanted recombination. Increasing V.sub.be and V.sub.cb to overcome these traps and reduce the resulting recombination, unfortunately, also reduces .DELTA.E.sub.v, which allows more hole injection from the base into the emitter. Thus, DC current gain decreases because the base to emitter current increases without a resulting increase in collector to emitter current.